1. Field of the Invention
This invention relates to a metallic fiber tow and more particularly to an improved method of making a fiber tow having fibers of plural diameters. This invention relates to an improved method of making a fiber tow having a diameter previously unobtainable in the prior art on a commercial basis. This invention also relates to a composite material comprising a fiber tow having fibers of plural diameters encapsulated within a polymeric material to form a two dimensional conductive layer.
2. Background of the Invention
The problems associated with electrostatic discharge and the damaging effects to sensitive electronic components have been well known in the prior art. A static charge can be generated by friction between two surfaces resulting in a substantial potential difference created between the two surfaces. A sensitive electronic component such as an integrated circuit or a circuit board that may come into proximity or contact with one of the statically charged surfaces can be damaged or destroyed by a static discharge from one of the statically charged surfaces.
A related problem exists relating to interference generated by electronic devices especially electronic devices encased in polymeric cases. This interference is commonly referred to as electromagnetic interference (EMI) generated in the kilohertz to gigahertz frequency range. In addition, many electronic devices encased in polymeric cases must be shielded from external electromagnetic interference (EMIT).
To overcome these separate but related problems, the prior art had used a variety of composite materials comprising a polymeric substrate and a conductive metal layer. These composite materials typically have used a continuous metallic coating deposited onto the polymeric substrate for creating an electrostatic shield commonly referred to as a Faraday cage. Others in the prior art have used a discontinuous metallic coating encapsulated in a polymeric laminate. One discontinuous metallic coating of the prior art encapsulated a metallic powder within a polymeric laminate while another discontinuous metallic coating of the prior art encapsulated metallic fibers within a polymeric laminate.
U.S. Pat. No. 2,215,477 to Pipkin discloses a method of manufacturing wires of a relatively brittle metal that consists of assembling a rod of the metal within a tube of a relatively ductile metal to form therewith a composite single assembly and successively drawing the assembly through a series of dies to form a composite wire element. A plurality of the wire elements are assembled within a tube of metal of the same character as that of the first-named tube to form therewith a composite multiple assembly, successively drawing the multiple assembly through a series of dies to reduce the same to a predetermined diameter, and then removing the ductile metal from the embedded wires of brittle metal.
U.S. Pat. No. 3,378,999 to Roberts et al discloses a metal yarn structure wherein the filaments are set under pressure while in a substantially nonelastic state to be free of residual torsion while having a preselected helical twist. The setting of the filaments in the helical configuration is effected by twisting the filaments in a matrix while concurrently effecting constriction thereof to fluidize the filaments and permit the setting thereof upon release of the constriction forces in the torsion-free helical configuration.
U.S. Pat. No. 3,540,144 to Roberts et al discloses a method of forming fine filaments formed of a material such as metal by multiple end drawing a plurality of elongated elements having thereon a thin film of lubricant material. The plurality of elements may be bundled in a tubular sheath formed of drawable material. The lubricant may be applied to the individual elements prior to the bundling thereof and may be provided by applying the lubricant to the elements while they are being individually drawn through a coating mechanism such as a drawing die. The lubricant comprises a material capable of forming a film having a high tenacity characteristic whereby the film is maintained under the extreme pressure conditions of the drawing process. Upon completion of the constricting operation, the tubular sheath is removed. If desired, the lubricant may also be removed from the resultant filaments.
U.S. Pat. No. 3,698,863 to Roberts et al discloses a metallic filament that has an effective diameter of less than 50 microns and is formed while surrounded by a subsequently removed sacrificial matrix. The filament has a preselected peripheral surface varying from substantially smooth to re-entrant and a preselected surface to volume ratio. The area of the filament also has a controlled non-uniformity along the length thereof that provides an acceptable dimensional tolerance. The metallic filament may be substantially one metal, bimetallic or tubular.
U.S. Pat. No. 3,977,070 to Schildbach discloses a method of forming a tow of filaments and the tow formed by the method wherein a bundle of elongated elements, such as rods or wires, is clad by forming a sheath of material different from that of the elements about the bundle and the bundle is subsequently drawn to constrict the elements to a desired small diameter. The elements may be formed of metal. The bundle may be annealed, or stress relieved, between drawing steps as desired. The sheath may be formed of metal and may have juxtaposed edges thereof welded together to retain the assembly. The sheath is removed from the final constricted bundle to free the filaments in the form of tow.
U.S. Pat. No. 4,118,845 to Schildbach discloses a method of forming a tow of filaments and the tow formed by the method wherein a bundle of elongated elements, such as rods or wires, is clad by forming a sheath of material different from that of the elements about the bundle and the bundle is subsequently drawn to constrict the elements to a desired small diameter. The elements may be formed of metal. The bundle may be annealed, or stress relieved, between drawing steps as desired. The sheath may be formed of metal and may have juxtaposed edges thereof welded together to retain the assembly. The sheath is removed from the final constricted bundle to free the filaments in the form of tow.
U.S. Pat. No. 4,371,742 to Manly discloses an absorptive shield for transmission lines, especially those tending to radiate electromagnetic wave lengths within a frequency range of from about 10.sup.6 to about 10.sup.10 hertz, especially 10.sup.7 to about 10.sup.10 hertz. The shields are flexible materials filled with ferromagnetic, or ferrimagnetic, powders of selected particle size and distribution.
U.S. Pat. No. 4,408,255 to Adkins discloses an electromagnetic shield comprising two portions in which the first portion consists of a magnetically permeable mat with a conductive sheet bonded to one side and an insulating sheet bonded to the opposite side. In a typical application, this first portion is positioned with the insulating sheet making contact to the underside of a printed circuit board. The second portion consists of a magnetically permeable mat with a conductive sheet bonded to each side. The mat is porous and one of the conductive sheets contains a plurality of openings to permit cooling air that is forced through the pores of the mat to pass through these openings. The conductive sheet containing the plurality of openings is positioned adjacent the components on the upper side of a printed circuit board to provide cooling as well as closely positioned shielding.
U.S. Pat. No. 4,664,971 to Soens discloses a plate or sheet-like article made of plastic in which very low contents of fine electrically conductive fibers are uniformly dispersed so as to make the articles conductive. It also relates to specific intermediate plastic products, referred to as grains, threads and granules, and the processes for manufacturing each of these products as well as the final conductive articles. The articles can be used as a suitable shielding against radio-frequency and high-frequency electromagnetic radiation or as antistatic plastic articles.
U.S. Pat. No. 4,785,136 to Mollet discloses an electromagnetic interference shielding cover for computer terminals or the like comprising a layer of woven metallic or metalized synthetic conductive fabric covering the computer terminal top side, bottom side, right side, left side, front side and rear side. The embodiments described provide full electrical and magnetic continuity throughout the shielding cover and can take the form of a free standing box-like rigid cover, or a fitted flexible cover. Woven metal or metalized conductive mesh conditioned for viewing enhancement and glare reduction is provided in an individual, framed section and connected over appropriate cut-out openings to allow continuity of electromagnetic shielding and visual access. The shielding cover may consist of a single enclosure or where appropriate, multiple enclosures connected by means of an electromagnetically continuous joint allowing console articulation.
U.S. Pat. No. 5,028,490 to Koskenmaki et al discloses a discontinuous metal/polymer composite, with a metal layer, formed from a plurality of fine metal strands, which may be used, for example, in static or EMI shielding. The metal layer comprises a plurality of fine metal strands provided on the substrate, the metal strands individually having a cross-section with an area of about 100 to 100,000 pm.sup.2 and the cross-section of the individual metal strands having a flat portion and an arcuate portion. The metal and polymer may be selected so that the composite is capable of being thermoformed without loss of electrical conductivity or transparency.
U.S. Pat. No. 5,137,782 to Adriaensen et al discloses a granular composite obtained by chopping a composite strand containing metal fibers, the fibers being embedded as bundles in a plastic and is to be used for the shaping of plastic articles. The metal fibers comprise hardened material that has been derived from an austenitic ferric alloy in which the austenite has been covered into martensite for at least 75 volume percent.
U.S. Pat. No. 5,165,985 to Wiste et al discloses a method of making a flexible transparent film providing electrostatic shielding by applying a plurality of thin conductive slivers to a sheet having a dimemsionally stable layer and a thermoplastic layer. The slivers form a two dimensional conductive network.
U.S. Pat. No. 5,226,210 to Koskenmaki et al discloses a metal/polymer composite comprising a polymeric substrate and a sintered mat of randomly-oriented metal fibers embedded therein, the fibers having a substantially circular cross-section and a diameter of about 10 to 200 Nm. The polymeric substrate is typically a thin, flexible sheet-like material having a pair of planar surfaces. The polymeric substrate is preferably thermoformable. If thermoformability is desired the metal will have a melting point of less than the thermoforming temperature of the polymeric substrate. The thermoformable metal/polymer composite of the present invention may be stretched to at least 20%, and often can be stretched at least 200% of its original dimensions, at least in certain regions, without loss of electrical continuity or EMI shielding properties. The present invention also provides a method of making a metal/polymer composite and a sintered mat of randomly-oriented metal fibers.
Although the aforementioned references have contributed to the art, the use of a plurality of thin conductive slivers to form a two dimensional conductive network has provided a substantial improvement to the problems referred to above.
The plurality of thin conductive slivers is formed through a cladding and drawing process wherein metallic wire is clad and drawn to reduce the diameter of the wire. A plurality of the drawn metallic wires are clad and drawn to further decrease the diameter. The cladding and drawing process is continued until each of the plurality of conductive slivers obtains the proper diameter. Typically, each of the plurality of thin conductive slivers has a diameter of 4 microns.
It is therefore a primary object of the present invention to further improve the method of making a multiple diameter metallic tow material having major diameter fibers and minor diameter fibers.
Another object of this invention is to provide an improved method of making multiple diameter metallic tow material having major diameter fibers and minor diameter fibers with the minor diameter fibers having a diameter previously unobtainable in the prior art on a commercial basis.
Another object of this invention is to provide an improved method of making multiple diameter metallic tow material having major diameter fibers and minor diameter fibers capable of being severed into uniform length to provide slivers of metallic wires having major and minor diameters.
Another object of this invention is to provide an improved method of making multiple diameter metallic tow material having major diameter fibers and minor diameter fibers capable of being severed into uniform length to provide slivers of metallic wires for making a composite material comprising the slivers of metallic fiber and a polymeric material.
Another object of this invention is to provide an improved method of making multiple diameter metallic tow material having major diameter fibers and minor diameter fibers capable of being severed into uniform length to provide slivers of metallic wires for encapsulation within polymeric material for providing an electrically conductive metallic layer therein.
Another object of this invention is to provide an improved method of making multiple diameter metallic tow material having major diameter fibers and minor diameter fibers capable of being severed into uniform length to provide slivers of metallic wires for encapsulation within polymeric material to provide an electromagnetic interference resistant layer.
The foregoing has outlined some of the more pertinent objects of the present invention. These objects should be construed as being merely illustrative of some of the more prominent features and applications of the invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or modifying the invention with in the scope of the invention. Accordingly other objects in a full understanding of the invention may be had by referring to the summary of the invention, the detailed description describing the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.